Wow, Apple finally approved Google’s own awesome iPhone application, free on App Store (Download). There were paid alternatives previously which didn’t do justice to Google’s service at all.
Check some tips out to get the most out of Google voice and hey, stop paying that $20 bill for unlimited texts, will you?!
Google’s official announcement can be read here…

UPDATE : Google Voice app for iPhone is now available on App Store!
Apple hates people bypassing the carrier’s features on the iPhone. Google’s Voice application which provides free texts and some other super cool features, all for free was never approved on the app store. Why? The carrier will be losing their share on SMS tax (20 cents for 140 bytes – really?). Smart people use unlimited texts add-on which costs $20 with AT&T. Smarter people use Google Voice! You are paying an awful lot of money for your cellular …

UPDATE : The issue has been fixed in the latest update released. Launch Cydia and update GV Mobile to v1.22.2 and you should be all set!
Sean Kovacs’ GV Mobile is no doubt the best native iPhone client available only on Cydia after Apple’s asinine decision to block all Google Voice related apps from the AppStore.

GV Mobile App suffered from some issues after Google Voice changes on the web front end. The two known issues being :

Invalid Credentials Error
Login Info not being saved (requiring username/password each time the app was launched)